Hash

Jenkins hash算法， memcached使用了此算法,普遍的被凯源项目所使用，是很好的ahsh算法之一。
Jenkins hash，可能是目前能看到的最好的hash算法之一，可以产生很好的分布，缺点是相比其他常见的hash算法更耗时。可以考虑用于hash表的open addressing实现上。如果想了解细节的话，可以去Bob Jenkins的站点看一看。

http://burtleburtle.net/bob/hash/

typedef  unsigned long int  u4;   /* unsigned 4-byte type */
typedef  unsigned     char  u1;   /* unsigned 1-byte type */

/* The mixing step */
#define mix(a,b,c) \
{ \
  a=a-b;  a=a-c;  a=a^(c>>13); \
  b=b-c;  b=b-a;  b=b^(a<<8);  \
  c=c-a;  c=c-b;  c=c^(b>>13); \
  a=a-b;  a=a-c;  a=a^(c>>12); \
  b=b-c;  b=b-a;  b=b^(a<<16); \
  c=c-a;  c=c-b;  c=c^(b>>5);  \
  a=a-b;  a=a-c;  a=a^(c>>3);  \
  b=b-c;  b=b-a;  b=b^(a<<10); \
  c=c-a;  c=c-b;  c=c^(b>>15); \
}

/* The whole new hash function */
u4 hash( k, length, initval)
register u1 *k;        /* the key */
u4           length;   /* the length of the key in bytes */
u4           initval;  /* the previous hash, or an arbitrary value */
{
   register u4 a,b,c;  /* the internal state */
   u4          len;    /* how many key bytes still need mixing */

   /* Set up the internal state */
   len = length;
   a = b = 0x9e3779b9;  /* the golden ratio; an arbitrary value */
   c = initval;         /* variable initialization of internal state  or c = 0x9e3779b9 can remove this canshu */

   /*---------------------------------------- handle most of the key */
   while (len >= 12)
   {
      a=a+(k[0]+((u4)k[1]<<8)+((u4)k[2]<<16) +((u4)k[3]<<24));
      b=b+(k[4]+((u4)k[5]<<8)+((u4)k[6]<<16) +((u4)k[7]<<24));
      c=c+(k[8]+((u4)k[9]<<8)+((u4)k[10]<<16)+((u4)k[11]<<24));
      mix(a,b,c);
      k = k+12; len = len-12;
   }

   /*------------------------------------- handle the last 11 bytes */
   c = c+length;
   switch(len)              /* all the case statements fall through */
   {
   case 11: c=c+((u4)k[10]<<24);
   case 10: c=c+((u4)k[9]<<16);
   case 9 : c=c+((u4)k[8]<<8);
      /* the first byte of c is reserved for the length */
   case 8 : b=b+((u4)k[7]<<24);
   case 7 : b=b+((u4)k[6]<<16);
   case 6 : b=b+((u4)k[5]<<8);
   case 5 : b=b+k[4];
   case 4 : a=a+((u4)k[3]<<24);
   case 3 : a=a+((u4)k[2]<<16);
   case 2 : a=a+((u4)k[1]<<8);
   case 1 : a=a+k[0];
     /* case 0: nothing left to add */
   }
   mix(a,b,c);
   /*-------------------------------------------- report the result */
   return c;
}